Building community takes time. You have to encourage participation, and sometimes kick start the conversation by asking questions yourself or requesting help from friends willing to ask or answer questions in the community.
When I helped start the AMA Boston blog, we recruited a number of bloggers from our volunteer pool, scheduling each blogger's post. We had some results, but not exactly what we were looking for. So we decided to move the community over to a Ning social media community, where those same volunteer bloggers started to post their articles on the social media site. As more people joined the site, those new community members started to post their own articles without being asked, or even being a member of the AMA. Once we had about 40 or so active people posting articles, we no longer needed the bloggers.
The AMA case study illustrates the early costs involved in starting a community, you have to do a lot of prior planning, and set the groundwork for building a successful community. The costs in setting up a community come early. Once you've succeeded in establishing a successful community you don't have to be as active in participating. Instead, you will want to give more power to members of the community to moderate and grow the community themselves. Though it does depend upon your goals and the rules operating in your industry, if you are going to run a successful community, it's important to remember the more power you give to people to gain benefits from the community through recognition, the more active they will be.